1. Field of the Invention
The present invention relates to plasma systems of the type in which a supersonic plasma stream is generated, and more particularly to plasma systems in which the plasma gun or generator has a plurality of electrodes of common polarity such as a segmented cathode in conjunction with an electrode of opposite polarity such as an anode.
2. History of the Prior Art
Supersonic plasma systems are well known as shown, for example, by U.S. Pat. No. 3,839,618 of Muehlberger, which patent issued Oct. 1, 1974 and is entitled "METHOD AND APPARATUS FOR EFFECTING HIGH-ENERGY DYNAMIC COATING OF SUBSTRATES". Such systems combine the introduction of an inert gas with an electrical potential difference at the electrodes to generate a plasma stream. The plasma stream establishes a transfer arc between the plasma generator or gun and a workpiece or target spaced a selected distance from the gun. The plasma stream may be used to heat the workpiece. The plasma stream may also be used to deposit a metallic coating on the workpiece such as where powdered metal is introduced into the plasma stream within the gun. The coupling of a vacuum or similar pumping source to an enclosure containing the plasma gun and the workpiece produces a low static pressure environment, resulting in a supersonic plasma stream. Such supersonic plasma systems have been found to be highly advantageous for many applications.
An improved supersonic plasma system is described in U.S. Pat. No. 4,328,257 of Muehlberger et al, which patent issued May 4, 1982 and is entitled "SYSTEM AND METHOD FOR PLASMA COATING". In the plasma system disclosed in the Muehlberger et al patent, a plasma gun is disposed within a closed chamber together with a workpiece. D.C. power supplies are coupled to the plasma gun and to the workpiece, and motion mechanisms are provided so that the gun and the workpiece may undergo various motions relative to one another. A powder feed mechanism is provided so that metallic powder can be injected into the plasma gun for spraying and deposition onto the workpiece. A lower end of the closed chamber downstream of the workpiece and the plasma gun is coupled through various filter and heat exchanger units to vacuum pumps for providing a low ambient pressure. The plasma gun contained at the upper end of the closed chamber is of conventional configuration and includes a single cathode and a single anode. As described in the patent a D.C. transfer arc power source coupled between the gun and the workpiece may be selectively switched in polarity to provide a reverse transfer arc where desired.
For certain applications of plasma systems, it has been found advantageous to provide a plasma generator or gun having plural or segmented electrodes of like polarity in conjunction with a common electrode of opposite polarity. Typically, the plural electrodes of like polarity are comprised of a segmented cathode which provides two or more separate cathodes in conjunction with a common anode. Examples of this are provided by Japanese Patent Publication No. 51(1976)-7556, of Mar. 9, 1976, and Japanese Patent Publication No. 61(1986)-230300, dated 1986. The Japanese patent publications describe plasma guns having segmented cathodes in which three separate cathodes are provided in conjunction with a common anode. Among other things, such a segmented electrode configuration enables metallic powder or other spray material to be introduced through a central injection port extending along the central axis of the plasma gun and the common electrode. This facilitates injection of such material when contrasted with the more conventional single cathode guns in which injection is made from the side of the gun, typically at an angle of 90.degree. or more relative to the central axis of the gun.
Segmented cathode plasma systems such as the systems described in the Japanese patent publications noted above are designed to operate with an ambient pressure at or close to atmospheric pressure. As such, the systems do not enjoy the substantial advantages that flow from operation of a plasma system at low ambient pressure such as is provided through use of a vacuum pumping system. Heretofore, however, segmented cathode systems have not been designed for use in a vacuum or similar low ambient pressure environment, probably because of the complexities of designing such a system so that it will even operate let alone operate efficiently. The presence of the plural cathodes with their individual arc chambers in conjunction with the common anode presents a number of complicated problems. Such problems further make adaptability to different operating conditions extremely difficult. Still further complications are presented by the central powder delivery system which must be carefully integrated with other portions of the multi-electrode system.
Accordingly, it would be desirable to provide a multi-electrode plasma system such as a segmented cathode system which may be operated in a vacuum or similar low ambient pressure condition. It would furthermore be advantageous to provide such a system which is adaptable or adjustable to different operating conditions and parameters, including in particular operation under relatively high power conditions.